Reinforced thermoplastic storage vessel manufacture

ABSTRACT

A method is disclosed for reinforcement of thin wall hollow thermoplastic storage vessels with one or more wraps of continuous fibers. This method requires thermal bonding between the reinforcement fibers and the outer surface of the thermoplastic storage vessel while the interior cavity of the storage vessel is being pressurized. The fiber wraps can also be oriented in spatial directions further resisting internal stress on the storage vessel walls when put in service.

[0001] This is a division of application Ser. No. 09/726,252 entitledMethod to Reinforce Thin Wall Thermoplastic Storage Vessels filed Nov.30, 2000.

BACKGROUND OF THE INVENTION

[0002] This invention relates generally to a method for reinforcement ofhollow thermoplastic storage vessels with one or more wraps ofcontinuous fibers and more particularly to a means for improved bondingbetween the applied fibers and the outer vessel surface for storagevessels having relatively thin walls.

[0003] In a co-pending application Ser. No. 09/327,003 entitled“Reinforced Thermoplastic Pipe Coupling” and filed Jun. 7, 1999 in thenames of David E. Hauber, Robert J. Langone and James A. Mondo which isalso assigned to the present assignee, there is disclosed a continuousfiber reinforced thermoplastic pipe coupling having improved resistanceto applied stress when used with pipe lengths being joined together. Thefiber reinforcement is aligned during placement in a particular mannerand placed at predetermined fiber angles dictated by mechanical forcesbeing applied such as by internal fluid pressure in the coupled pipelengths. Said already known method for construction of said reinforcedthermoplastic pipe coupling includes a controlled directionalorientation of the fiber component to enable the fiber placement to befixed for maximum effectiveness in withstanding the particular stressbeing generated when the joined together pipe lengths are customarilyused for the transfer of pressurized fluids. Since the fiber materialscurrently used in this manner are generally stronger than the polymermatrix compositions also being employed, the overall strength producedin the composite member depends largely upon the fiber placementdirection for the particular end product. The fiber reinforced coupleris thereby only as strong as the spatial direction of the includedfibers with respect to the direction of the internal stress when appliedto said member. Thus, when the fiber reinforced coupler is stressed byinternal fluid pressures in the direction of the fiber placement, theapplied load is withstood primarily by the included fibers and thecoupler strength in resisting such stress is at a maximum value.Conversely, when the composite member is stressed in a perpendiculardirection to the fiber direction, the applied force must necessarily beresisted primarily by the polymer matrix so that the coupler strength isat minimum. The relative amounts of the individual stresses beingapplied to the fiber reinforced coupler must also necessarily beconsidered for proper fiber placement direction. For an externallyunconstrained installation of said previously disclosed pipe couplings,such as encountered with above ground pipe installations, the appliedloads can be examined by treating the joined pipe lengths as a pressurevessel. From such analysis it was found that the stress applied to thepipe wall in the hoop direction is twice an amount as the applied stressin the pipe's axial direction. Employing well recognized shell theorycalculation, it was further found that a fiber angle of 55 degrees wasneeded to balance these applied loads assuming 90 degrees to be in thepipe hoop direction and 0 degrees to be aligned in the direction of thepipe longitudinal axis. For constrained pipe installations, however,such as in-ground or having the pipe ends being held there, there canonly be need for resisting hoop stress. Accordingly, fiber placement ator near a 90 degree angle with respect to the longitudinal pipe axis wasdictated while further recognizing that some angle less than 90 degreesmay only be achievable with the fiber winding in the customary manner.The entire contents of said referenced co-pending application are herebyspecifically incorporated into the present application.

[0004] It can readily be appreciated that thermoplastic storage vesselsundergo similar internal stress when being utilized. Accordingly, theeffectiveness of fiber reinforcement for thermoplastic storage vesselswill also depend to a considerable degree upon the same factorspreviously considered with respect to said reinforced thermoplasticcouplings. For example, a thermoplastic storage vessel having acylindrical configuration can generally have the fiber wraps applied ina hoop direction for maximum reinforcement whereas a spherical storagevessel will understandably have the fiber placement angle varied indifferent spatial directions. It has now been found, however, thatthermal bonding the reinforcement fibers to the outer surface of thethermoplastic storage vessel in the same manner previously employed forreinforcement of said thermoplastic pipe couplings produces inferiorresults. Specifically, the previously employed bonding method providedsufficient thermal expansion of the thermoplastic inner coupling memberwhen being carried out that an effective thermal bonding with theapplied fiber reinforcement took place. This does not reliably occur forvarious shaped thermoplastic storage vessels having a lesser wallthickness. It thereby becomes necessary for said relatively thin wallstorage vessels to adopt an improved thermal bonding procedure for thefiber reinforcement to have the desired effectiveness.

[0005] It is an important object of the present invention, therefore, toprovide a novel method to reinforce thin wall thermoplastic storagevessels with one or more wraps of applied continuous fiber.

[0006] It is still another object of the present invention to provide anovel method to secure the applied fibers to the outer surface of a thinwall thermoplastic storage vessel so as to better resist internal stresswhen the storage vessel is in use and prevent delamination when pressureis released.

[0007] Still another object of the present invention is to provide anovel method for reinforcement of a thin wall thermoplastic storagevessel which includes a plurality of continuous juxtapositioned fibersbeing reliably secured to the outer surface of said storage vessel so asto be aligned in a predetermined spatial direction resisting appliedinternal stress during vessel use.

[0008] These and still further objects of the present invention willbecome more apparent upon considering the following more detaileddescription of the present invention.

SUMMARY OF THE INVENTION

[0009] It has now been discovered by the present applicant that acontemporaneous pressurization of the internal cavity in a thin wallthermoplastic storage vessel while the applied reinforcement fibers onthe outer surface of said storage vessel are being thermally bondedthereto overcomes the problem previously experienced with inadequatejoinder of said reinforcement means. The internally applied pressure isseen to avert buckling or wrinkling of the thin storage vessel wallwhile being heated sufficiently for joinder between the reinforcementfibers and the outer vessel surface thereby enabling a sufficientbonding action therebetween. Internal pressurization of the storagevessel can thereafter be discontinued in the present reinforcementmethod allowing the fiber wrapped storage vessel to cool upontermination of said thermal bonding action. Accordingly, the presentmethod to reinforce said type thin wall hollow storage vessel compriseswrapping a plurality of continuous juxtapositioned reinforcement fibersformed with a material composition selected from the group consisting ofceramics, metals, carbon and organic polymers while in an unbondedcondition about the outer surface of said storage vessel, heating theouter vessel surface sufficiently to cause thermal bonding between thereinforcement fibers and said outer fiber wrapped vessel surface,contemporaneously pressurizing the interior cavity of said rotatingfiber wrapped storage vessel with a coolant medium during said heatingstep, and allowing the fiber wrapped storage vessel to cool uponterminating said heating step before discontinuing pressurization of thevessel interior cavity. Various liquid or gaseous coolants can beemployed in the present method to include water, air, nitrogen or thelike, while removal of said coolant medium from the storage vessel afterbeing heated during the present thermal bonding step can assist finalcooling of said reinforcement fiber wrap vessel. Thermal bonding in thepresent method involves some melting of the thermoplastic materialsbeing employed so that melting of the thermoplastic outer vessel surfaceoccurs which can be accompanied by melting of a thermoplastic matrixincluded in the applied fiber reinforcement. Accordingly, a softening ormelting action takes place during the present thermal bonding stepbetween the outer surface of the thermoplastic storage vessel and anythermoplastic polymer materials serving as the matrix composition inselected preformed tape embodiments having the continuous reinforcementfibers thereafter becoming permanently bonded therein.

[0010] The herein defined fiber reinforcement method understandablyenables a wide variety of fiber materials to be selected as previouslypointed out. Thus, a reinforcement fiber material can be selected fromthe aforementioned class of suitable materials so long as it ismechanically stiffer than the selected thermoplastic vessel polymer andhas a glass transition or melting temperature higher than the surfacetemperature of the thermoplastic vessel during use. Selected polymerfibers can understandably include continuous bare filaments andcommingled continuous fibers which can be wetted by polymer melt flow inthe above described heat bonding procedure. For selection of a suitablepreformed continuous fiber material or prepreg tape having a matrixformed with a thermoplastic polymer, said matrix polymer is desirablychosen to have a softening or melt temperature equal to or lower thanthe softening temperature of the selected vessel polymer. Any suitableheating source can be used in the present method to reliably bond theapplied fiber reinforcement to the outer thermoplastic vessel surface.Contemplated heat sources include but are not limited to inert gases,oxidizing gases and reducing gases, including mixtures thereof, infraredheating sources, such as infrared panels and focused infrared means,conduction heating sources such as heated rollers, belts and shoedevices, electrical resistance heating sources, laser heating sources,microwave heating sources, RF heating sources, plasma heating sourcesand ultrasonic heating sources. An external flame heating sourceprovides economical heating with high-energy densities and with the gasburner or burners being suitably designed so as to heat the outercircumference of the fiber wrapped thermoplastic vessel. In a preferredembodiment, the wrapped storage vessel is rotated about the selectedheat source while having the interior cavity of said storage vesselbeing subjected to a pressurized condition. The applied pressure candesirably produce some radial expansion of the storage vessel wallthereby further enhancing the thermal bonding action taking place. Theapplied pressurization can also be initiated prior to said heating stepin the present method with applied pressures of ten pounds per squareinch or more having been found effective.

[0011] The fiber alignment selected in the present method can also varywith the particular shape of the thermoplastic storage vessel beingreinforced in said manner. Thus, a cylindrically shaped thermoplasticwater heater can have one or more wraps of the reinforcement fibersaligned in a hoop or helical direction whereas a spherical thermoplasticstorage vessel for the same use can understandably be wrapped indifferent spatial directions. A means of preserving the fiber alignmentin the present method until the melted polymer in physical contacttherewith again becomes solid can require that said fibers be subjectedto appropriate applied mechanical force during the thermal bondingaction. Such manner of fiber placement can be carried out by employingexternal tension winding means to guide the fiber reinforcement whilebeing wound around the outer vessel surface. An alternate means forretaining the fiber alignment is a compaction roller to apply mechanicalpressure to the heated fiber and polymer materials while being bondedtogether. Use of a compaction roller in such fiber placement can applyan external compaction force with zero tension force being applied ifdesired although it is within contemplation of the present invention forboth forms of external mechanical energy to be employed together whenfound beneficial. Another advantage of compaction roller use is theability to orient such means in any spatial direction enabling fiberplacement at a predetermined fiber angle dictated by the contour of theparticular storage vessel being reinforced in said manner. Thus, acylindrical shaped thermoplastic pressure vessel can have one or morewraps of the reinforcement fibers aligned in a hoop or helical directionwhereas a spherical thermoplastic storage vessel for such use can bewrapped in different spatial directions.

[0012] Following termination of said thermal bonding step in the presentmethod, the fiber wrapped storage vessel can be allowed to cool in theambient atmosphere. Such cooling can be carried out in various ways toinclude removal of any pressurizing liquid or gas coolant heated duringthe thermal bonding procedure as well as actively cooling with anapplied coolant medium. The completed fiber reinforcement can now serveto enable sufficiently higher operating pressures in said storagevessels than otherwise permissible. Employment of the present methodupon an otherwise conventional thermoplastic pressure vessel having aclosed end cylindrical configuration has produced this result.Additionally, an outer protective or decorative coating to include heatshrinkable tubing, wrap or extruded coatings and the like can be appliedto said fiber reinforced thermoplastic storage vessel in a conventionalmanner for protection of the fiber reinforcement from environmental ormechanical damage and/or corrosion.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a block diagram illustrating successive processing stepswhich can be employed in carrying out the method of the presentinvention.

[0014]FIG. 2 is a side view for a representative thermoplastic storagevessel being reinforced according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Referring to the drawings, FIG. 1 is a block diagramrepresentation illustrating the sequence of processing steps employedaccording to the present invention for fiber reinforcement of arepresentative thermoplastic storage vessel having a closed endcylindrical configuration. The depicted fiber reinforcement process 10employs a typical six inch diameter, thirty-two inch long thermoplasticliquid container 12 having a 0.14 inch wall thickness which has one ormore wraps of the thermoplastic reinforcement fibers 14 helically woundabout the outer cylindrical surface of said storage vessel. One or moretie wraps 16 of said thermoplastic reinforcement fibers can also besubsequently applied in the hoop direction for the purpose of carryingthe radial stress in the cylindrical pressure vessel. Said fiber wrappedvessel 18 next undergoes thermal bonding of the applied fiberreinforcement to the outer vessel surface. In a preferred embodiment,the fiber wrapped vessel is rotated about its central axis 20 whileheating the outer vessel surface with a conventional heat source 22.Heating of the fiber wrapped vessel in said manner produces some meltingof the outer vessel surface which upon vessel cooling retains theoriginally applied spatial orientation of said fibers. During saidheating step the hollow interior cavity of said fiber wrapped storagevessel 18 is pressurized 24 by various means to avoid any significantwrinkling or collapse of the vessel wall that could understandably detera fully bonded condition for the applied fiber reinforcement. Internalpressurization of the storage vessel can be initiated before thermalbonding of the fiber reinforcement while thereafter being discontinuedwhen the thermal bonding step has been completed and the reinforcedstorage vessel then being allowed to cool 26°. Terminatingpressurization of the storage vessel 28 can also be carried out invarious ways. To further illustrate a suitable vessel pressurization inthe present method, the interior cavity of the fiber wrapped storagevessel 18 can be filled with a liquid coolant, such as water, glycol,alcohol and the like before the above described heating step is begun aswell as thereafter being removed from the storage vessel after becomingheated during said processing step. Alternately, the interior cavity ofsaid storage vessel 28 can be actively cooled with a suitable gaseouscoolant to include air, nitrogen or other inert gas while the thermalbonding step is being carried out and with said cooling action beingdiscontinued when the reinforced storage vessel is thereafter allowed tocool. Active cooling of the fiber wrapped storage vessel in said mannerat a pressure of 10 PSI or more has been proven satisfactory in thepresent method.

[0016] As herein pointed out, the fiber direction of the underlyingfiber layers for the illustrated cylindrical storage vessel is dictatedprimarily by the ability of said reinforced storage vessel to withstandinternal fluid pressures when such vessel is put into service. It canreadily be appreciated, however, that other storage vessels having adifferent shape, such as a sphere, can have the fiber alignment in anoverall hoop direction for better resistance to internal fluid pressuresduring use. Additionally, the continuous fiber reinforcement can beapplied in the present method by various means. A selected amount oftension can be exerted upon the continuous fibers when being applied toassist with retention of the predetermined or juxta-positioned fiberangle with respect to the vessel longitudinal axis in the hereinillustrated embodiment. Similarly, a mechanical compaction force exertedupon said fibers during initial placement or subsequent thermal bondingcan be employed for this purpose. A wide variety of thermoplasticpolymers can also be selected as the material of construction forstorage vessels being reinforced according to the present method.Suitable organic polymers include but are not limited to polyethylenesuch as high density polyethylene and medium density polyethylene,polypropylene, polyphenylene sulfide, polyetherketoneketone, polyamide,polyamideimide and polyvinylidene difluoride. A similar wide variety ofmaterials are found suitable as the fiber reinforcement in the presentmethod to again include but not be limited to ceramics, metals, carbonaramid and other organic polymer fibers having softening temperaturesabove that of the storage vessel in use and glass compositions such as Etype and S type glasses. Moreover, said fiber materials can also beapplied in various structural forms to include a parallel alignment ofthe bare fibers and conventional fiber tapes having the continuousparallel oriented fibers bonded together in a thermoplastic polymermatrix. The optional use being made of tie layers 16 in the presentlyillustrated embodiment can also serve to help retain the juxtapositionedspatial orientation of the applied fiber reinforcement when selectedthermoplastic polymer materials being employed are not miscible duringthe heating step.

[0017]FIG. 2 is a side view for a representative thermoplastic storagevessel being reinforced according to the present invention. Moreparticularly, the depicted cylindrical thermoplastic storage vessel 30is repeatedly illustrated during each processing step described in thepreceding preferred embodiment. As shown, said storage vessel 30comprises an elongated thermoplastic cylinder 32 having a closed end 34and an open end 36 fitted with a conventional inlet coupling 38. Thereis next depicted the manner whereby the continuous reinforcement fiber40 is deposited on the outer surface 42 of the rotating thermoplasticstorage vessel in a helical pattern 44 while also being subjected to atensile force being applied in the customary manner. The next processingstep being illustrated depicts further rotation of the fiber wrappedstorage vessel 46 while additional fiber wraps 48 are applied in a hoopdirection enabling better retention of the underlying reinforcementfiber 40. The still further depicted processing step in the hereinillustrated method of fiber reinforcement demonstrates the heating stepbeing employed to cause thermal bonding between the applied unbondedreinforcement fibers and the outer surfaces of said storage vessel. Indoing so, a conventional heat source 50 positioned in relatively closeproximity to said fiber wrapped storage vessel 46 supplies the neededthermal energy during said bonding procedure and which is furtheraccompanied by having the interior cavity 52 of said fiber wrappedstorage vessel pressurized with a selected liquid cooling medium 54while said thermal bonding step is being carried out. Following saidlatter procedure, the reinforced storage vessel 56 is allowed to cool inthe ambient atmosphere which further include removal of the pressurizingfluid after sufficient time has elapsed for solidification of thepolymers thermally bonded together.

[0018] It will be apparent from the foregoing description that a broadlyuseful and novel method has been provided to reinforce thin wallthermoplastic storage vessels with one or more wraps of appliedcontinuous fiber. It will be apparent, however, that variousmodifications can be made in the disclosed process without departingfrom the spirit and scope of the present invention. For example, it iscontemplated that some heating of the unbonded reinforcement when beinginitially applied to the outer surface of the storage vessel can assistin having the fiber conform more closely to the particular contours ofthe vessel surface. Likewise, it is contemplated that other organicpolymers, other vessel shapes and other processing equipment than hereinspecifically disclosed can be substituted in carrying out the presentmethod. Consequently, it is intended to cover all variations in thedisclosed reinforcement method which may be devised by persons skilledin the art as falling within the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:
 1. An apparatus to mechanically reinforce a thin wall hollowstorage vessel formed of a solid thermoplastic organic polymer whichcomprises: (a) helical winding means to wrap the outer surface storagevessel with at least one wrap of continuous reinforcement fibersselected from the group consisting of ceramics, metals, carbon andorganic polymers, (b) a heat source to thermally bond the appliedreinforcement fibers to the outer surface of said storage vessel, and(c) external fluid means to pressurize the hollow cavity of said storagevessel while thermal bonding of the reinforcement fibers is beingcarried out.
 2. The apparatus of claim 1 wherein the external coolantmeans employs a gaseous medium.
 3. The apparatus of claim 1 wherein theexternal coolant means employs a liquid medium.